1. Field of the Invention
This invention generally relates to a method and apparatus for the electrohydrodynamic assembly of two- and-three-dimensional colloidal crystals on electrode surfaces.
2. Related Art
The construction of materials with structural features existing on the 1-1000 nanometer (nm) size scale, is a rapidly emerging area in materials science. Such nanostructured materials, particularly multilayered thin film composites known as nanolaminates, exhibit remarkably different macroscopic properties than those of more conventionally engineered materials having structural features in the micrometer size range or greater. Examples of these nanostructured materials include metal-metal and ceramic-metal compositionally modulated nanolaminates for applications as diverse as high temperature gas turbine jet engines, soft x-ray and extreme ultraviolet mirrors, as well as magnetic materials for high-density magnetic recording and magneto-optical data storage and retrieval.
Moreover, it is known that nested levels of structural hierarchy in composite materials can impart vastly superior properties over homogeneously structured materials. Such design features are readily exploited in biological materials (e.g., bone, abalone shell, deer antler and muscle tissue) where subtle differences in structure, over various length scales, give rise to superior performance characteristics.
Although nanostructured materials display considerable potential, their development is currently limited the inability to conveniently and economically assemble such materials in large quantities, preferably under ambient conditions. Due to the intrinsic dimensional limitations of mechanical forming, pattern formation in man-made materials has heretofore been restricted to length scales larger than a few tens of microns. Nanolaminates have been produced by molecular deposition techniques, which utilize individual molecules as building blocks to form higher order structures, but these procedures are cumbersome, costly and usually only produce small quantities of material.
Another process for assembling designed structures is by electrophoretic deposition. The phenomenon of electrophoresis was first observed in 1807 by F. F. Ruess (Mem. Soc. Imp. Natuv. Mouscou, 1809, 2, 327) by passing an electric current through a suspension of clay in water, the clay particles immigrating towards the anode. Electrophoretic deposition of colloidal particles at electrodes has been used as a manufacturing technique for coating metal compounds. Metals, oxides, phosphores, inorganic and organic paints, rubber, dielectrics, superconductors and glasses have all been deposited via this technique using both aqueous and non-aqueous media. However, work in this area has been concerned with measuring the deposition rate, and achieving the maximum thickness and porosity of these films. By contrast, little attention has been devoted to the microscopic dynamics that give rise to the resulting morphology of the deposited layer. Indeed, it has generally been assumed that the dynamics of particle layer formation in these systems are identical to those which occur during particle sedimentation. This has been described by Hamaker, H. C. and Verwey E. J. W. (Trans. Faraday Soc. 1940, 36, 80) as resembling the force of gravity on particles in a container. Moreover, it has also been generally assumed that electrophoretically deposited layers are highly porous, resulting from a high degree of colloidal coagulation with the electrode which leads to poor packing efficiencies. Additionally, a key problem is the control of layer thickness.
The assembly of colloids into crystalline structures has heretofore been achieved by dispersing monosized colloids into solvent and manipulating either particle-particle interaction forces or entropic effects. The formation of these types of crystalline structures has proved to be difficult to regulate externally and cumbersome to confine to two dimensions. Similarly, protein crystallization has also proved to be difficult to regulate and is currently the ratio-determining step in the structure determination of biologically important proteins.
F. Richetti, J. Prost and N. A. Clark, in Electric Field Effects in Polyball Suspensions, examined the attractive reaction when an AC electric field induces motion between like charged polystyrene spherical "balls" to form two-dimensional particle clusters. While the explanation for the particle behavior acknowledges a lack of a detailed understanding, it does express a belief that the attractive force is frequency dependent. At low frequencies, the particle interactions are found to involve the field induced motion of the small counterion clouds surrounding each polystyrene ball as well as the electrohydrodynamic flow of the suspending fluid. At frequencies above a few kilohertz, the particles seem to attract each other via induced dipole fields.
Langmuir 1993, 9 3408,3413, M. Geirsig and P. Mulvaney, in preparation of ordered colloid monolayers by electrophoretic deposition discussed electrophoretic deposition and its usefulness as a technique for examining the surface chemistry of an ordered, gold colloidal monolayer. Micrographs indicate that an ordered monolayer, formed by electrophoretic deposition at field strengths of less than 1 volt per centimeter, is in fact built up of a large number of smaller crystalline domains, each domain containing 50 to 200 particles in the form of hexagonally closely packed colloidal particles. Transmission electron microscopy reveals that "grain boundaries" tend to form at particles which are either aspherical or too large to fit into the lattice and that such grain boundaries limit the formation of monolayers because they exacerbate the tendency of the monolayers to tear as they are removed from the aqueous solution after deposition.
None of the previous efforts in this field disclose all of the benefits of the present invention, nor does the prior art teach or suggest all of the elements of the present invention.